1. Field of the Invention
The present invention relates to a capacitor charging circuit, and particularly to a control method for a self-exciting capacitor charging circuit.
2. Description of the Related Art
For the purpose of generating higher voltage than the input voltage, step-up capacitor charging circuits are widely used in various electronic devices. Such a step-up capacitor charging circuit includes a switching element and an inductor or a transformer. With such an arrangement, the switching element is turned on and off in a time divisional manner so as to generate back electromotive force in the inductor or the transformer, thereby boosting the input voltage, and outputting the voltage thus stepped up.
When a switching transistor is turned off, an electric current flows through the primary winding of the transformer, thereby storing energy in the transformer. When the switching transistor is turned off, the energy thus stored in the transformer is transferred to an output capacitor in the form of a charging current via a rectifier diode in the secondary winding, thereby raising the output voltage. After the transfer of the energy stored in the transformer to the output capacitor, the current that flows through the rectifier diode becomes zero.
As a method for controlling an insulated-type capacitor charging circuit, a self-exciting control method without involving an oscillator is known, whereby the primary winding or the secondary winding of a transformer is monitored, and the ON/OFF control operation for the switching transistor is performed according to the state thus monitored.